Maybe you can't ask the TOCs but you could perhaps infer that some of the informed replies on here come from contributors much closer to the actual data than you or I.
Thank you.
I'll elaborate a little further just to clarify things a bit more without going into too much detail. Most rejected claims tend to fall into several common categories, eg. mistyped/selected stations (drop-down box, etc), misidentified train ID, misidentified dates, insufficient delay. These are all surprisingly common errors.
Some TOCs now also use automated systems to process initial claims, so any incorrect detail submitted can result in an automatic rejection, whereas claims processed by people tend to get a second look to pick out any obvious mistake, but that more often happens at an escalated stage now, where the customer provided more details about their journey and a more comprehensive assessment can be made, including second-guessing the date they actually meant, which I have had to do numerous times before personally. This also means an upheld appeal does not automatically mean the original rejection was incorrect.
That by and large works well.
Where there tends to be an issue is when the timetable had changed, and an originally intended service no longer existed. This is where evidence of the intended itinerary (in whatever readable format available) can be helpful to caseworkers. The legal position on whether a delay should be measured against a revised timetable or the original (no longer existent) timetable is also a tad vague AFAIK, even though there are various claims on the forum on what the supposed legal position is (which is a discussion for another thread), so I won't claim to know exactly what the answer is.
I won't get into the validity of claims surrounding split tickets either as there seems to be a bit of inconsistency surrounding how they are handled. Clearer guidelines from RDG would help, provided an agreement amongst TOCs is reached. Both of these issues constitute a very small proportion of claims, and in any case, if the amount involved is relatively small, often the claim is paid regardless as having to deal with it a third time, or possibly more, can be more expensive in terms of resources required.
What is more an inherent issue in the Delay Repay system in my opinion is that many people won't bother claiming small amounts. I don't do it myself because I don't consider spending 5 minutes claiming £1.50 is a good use of my time. With the introduction of Delay Repay 15, these small amounts are becoming more common.